1. Field of the Invention
The present invention relates to a radio frequency signal folding-back transmitting/receiving circuit for testing the operation of a radio transmitting/receiving apparatus. The present invention also relates to a radio transmitting/receiving apparatus having such a circuit.
2. Description of the Related Art
In recent years, to allow users to use a communication line at a low cost, it is desired to improve the efficiency of a maintenance operation periodically or non-periodically performed after a communication apparatus is installed. Thus, it is required to easily perform characteristic tests for the communication apparatus in the state in which it is operating.
FIG. 1 is a block diagram showing the structure of an opposite communication system having two radio transmitting/receiving apparatuses oppositely disposed. With reference to FIG. 1, an example of a system that allows a characteristic test to be effectively performed will be described.
Referring to FIG. 1, in a local station radio transmitting/receiving apparatus 1, a multiplexed signal is input from, for example, a public telephone line to a local station transmission signal input terminal 10. A local station transmitter 3 amplifies the power of the multiplexed signal. A radio wave corresponding to the amplified signal is transmitted from an antenna through a local station branching filter 5. In addition, a local station receiver 4 receives a radio wave from the antenna 2 through the local station branching filter 5. For example, a multiplexed signal corresponding to a transmission signal is output from a local station reception signal output terminal 11.
On the other hand, referring to FIG. 1, in an opposite radio transmitting/receiving apparatus 6, a multiplexed signal is input from for example a public telephone line to an opposite station transmission signal input terminal 12. An opposite station transmitter 8 amplifies the power of the multiplexed signal. A radio wave is transmitted from an antenna 2 through an opposite station branching filter 7. In addition, an opposite station receiver 9 receives a radio wave from the antenna 2 through the opposite station branching filter 7. For example, a multiplexed signal corresponding to a transmission signal is output from an opposite station reception signal output terminal 13.
In the opposite communication system (in particular, a radio communication system using a microwave circuit routed nationwide), a transmission side telephone unit is connected to a reception side telephone unit through a transmission side subscriber line exchange, a microwave repeating station, a repeating exchange, and a reception side subscriber line exchange. The transmission side telephone unit transmits a signal of audio, data, video, or the like to the receiving side telephone unit through a designated telephone network. Thus, a radio repeating station is used as one important elements of the network.
Conventionally, an indoor radio transmitting/receiving apparatus and an outdoor antenna are connected with a waveguide capable of conducting a radio signal frequency of the equipment signal or the like. As the radio frequency rises, the power loss of the communication apparatus increases. In addition, installation cost of higher frequency increases. Thus, a transmission frequency converting portion and a reception frequency converting portion of the radio transmitting/receiving apparatus are disposed adjacent to the antenna to omit a connecting portion of waveguide. Consequently, the installation costs and power loss can be decreased.
In the opposite communication system of the radio transmitting/receiving apparatuses shown in FIG. 1, when a radio frequency signal folding-back test is performed for the local station radio transmitting/receiving apparatus 1, a radio frequency signal folding-back test circuit is disposed in the radio transmitting/receiving apparatus shown in FIG. 2A. In the radio frequency signal folding-back test circuit, a frequency shifter 14 is disposed between the local station antenna 2 and the local station radio transmitting/receiving apparatus 1 so as to perform a radio frequency signal folding-back test of the local station radio transmitting/receiving apparatus 1. The frequency shifter 14 converts a transmission frequency into a reception, frequency.
In FIG. 2A multiplexed signal that is input to the local station transmission signal input terminal 10 is compared with a multiplexed signal that is folded back by the frequency shifter 14 to the local station reception signal output terminal 11. When the same multiplexed signal is correctly obtained, it can be determined that the local station radio transmitting/receiving apparatus is operating normally. When the same multiplexed signal is not correctly obtained, it can be determined that the local station radio transmitting/receiving apparatus is abnormal. Once detected, a defective portion can be located and corrected.
However, in the folding-back test shown in FIG. 2A, the local station antenna 2 and the frequency shifter 14 must be manually mounted and dismounted. The folding-back test cannot be easily performed because of is manual nature. In addition, this type of test interrupts the transceiver operation.
Alternatively, in a folding-back test circuit for a radio transmitting/receiving apparatus is shown in FIG. 2B, a frequency shifter 14 is disposed in the local station transmitting/receiving apparatus 1. A transmission side directional coupler 15 is disposed between a local station transmitter 3 and a local station branching coupler 5. In addition, a reception side directional coupler 16 is disposed between a local station transmitter 3 and a local station branching filter 5. A signal is folded back from the local station transmitter 3 to the local station receiver 4 through the directional couplers 15 and 16. In this structure, it is not necessary to mount and dismount the local station antenna 2 and the frequency shifter 14. In addition, the same radio frequency signal folding-back test circuit (not shown) may be disposed in the opposite station radio transmitting/receiving apparatus 6 shown in FIG. 2B.
However, in the structure shown in FIG. 2B, since a transmission signal of the opposite station radio transmitting/receiving apparatus 6 is always received, the output signal of the opposite station transmitter 8 should be turned off. In other words, to perform a radio frequency signal folding-back test for the local station radio transmitting/receiving apparatus 1, a transmission signal of the opposite station should be manually turned off. A manual turnoff is prescribed in case the opposite station receiver 9 becomes defective. If an automatic shutoff signal apparatus 6 gets defective, even if a signal that causes the off is transmitted from the local station radio transmitting/receiving apparatus 1 to the opposite radio transmitting/receiving apparatus 6 with a defective receiver 9, the output signal of the opposite station transmitter 8 cannot be turned off.
As an application of the folding-back test circuit shown in FIG. 2B, an attenuator is disposed between the local station branching filter 5 of the folding-back test circuit and the receiver side directional coupler 16. When a radio frequency signal folding-back test is performed, the attenuation amount of the attenuator is set to maximum value. Thus, the transmission signal of the opposite station radio transmitting/receiving apparatus 6 is not transmitted to the local station receiver 4.
In the radio frequency signal folding-back test method, since the insertion loss of the attenuator is large, noise figure (NF) as a performance index of the receiver deteriorates. When the attenuator is manually disposed, even if the insertion loss can be suppressed, the manual operation is required.